The present invention relates generally to the task of providing fossil fuel energy sources from a remote location to a local site for use. In particular, the present invention is drawn to a safe, environmentally sound method for transporting hydrocarbons from their remote, natural points of origin to local sites so that the energy content of such fuels can be used. More particularly, the present invention is drawn to a method of converting natural gas at a remote site, using a modified Fischer-Tropsch process, to produce non-volatile, long chain hydrocarbons, i.e., paraffins (C.sub.20 -C.sub.36) at the remote site, transporting the paraffins via rail, ship, or cargo air plane to a local site, and then further processing and/or refining the paraffins via distillation, cracking, or combining with other hydrocarbon feedstocks to produce fuel products at the local site for use.
Current methods for transporting certain hydrocarbon fuels via land or across the oceans present certain health hazards and environmental dangers.
Crude oil and refined fuel oil are commonly transported between continents by supertankers holding millions of gallons. Even with segmented tanks and double hull construction, unnatural disasters can occur when, due to human error or natural forces, the supertankers used to transport the oil are damaged.
In February 1996, an oil tanker ran aground off the coast of Wales in an environmentally endangered area, spilling several hundred thousand gallons of oil into the Atlantic Ocean.
The EXXON VALDEZ spill off of the coast of Alaska is one of the most vivid examples of the devastating effect that spilled fuels can have on the environment. Years following the disaster, the coastline was still being cleaned and the ecologies are still recovering as best they can.
Huge reserves of natural gas are found in many regions of the world, but they are frequently located in remote areas far from the actual centers of consumption. Traditional pipeline costs can account for as much as one-third of the total natural gas cost. Thus, there are tremendous strategic and economic incentives to provide on-site gas conversion to liquids.
Liquefied natural gas is another common fuel that is transported through tankers, trucks and pipelines and stored in large capacity tanks. In order to more efficiently transport and store natural gas, it is liquefied by reducing its pressure and temperature. Liquefied natural gas explodes easily, and it must be kept refrigerated and pressurized to remain liquefied. Any accidental leak of liquefied natural gas instantly begins to boil, gaining heat from the surrounding air, ground, building, or from whatever it may come into contact.
Natural gas is a combination of highly flammable hydrocarbon gases. Natural gas is normally composed of about 80% to 95% methane. When liquefied natural gas boils, as during a leak, it becomes even more volatile, and subject to rapid ignition and explosion from the slightest spark.
Liquefied natural gas poses even greater difficulties and dangers to the people and ecologies where it is stored and transported than crude oil. Precautions are taken to avoid explosions and fires, but disasters involving liquefied natural gas have been documented.
For example, in October 1944, a liquefied natural gas storage facility burst, releasing 2 million gallons of liquefied gas into streets and sewers of Cleveland, Ohio. The gas was easily ignited, burned and exploded, killing 131 people and razing 29 acres of developed property.
In February 1973, a liquefied natural gas tank on Staten Island, N.Y. exploded, killing 40 people. A pipeline at the same facility exploded in 1994.
A liquefied natural gas explosion occurred while workers were loading fuel trucks at a Hammond, Ind. site in February 1990. Two people were killed, several others were injured and the town was evacuated.
It is thus reasonable to seek transport methods for fossil fuels which are less vulnerable to danger.
However, it is generally accepted in the fuel production and refining industry that refining processes for fossil fuels should work to maximize the usable end product fraction at nearly every stage.
As discussed in The Encyclopedia of Energy Technology and the Environment, Vol. 2, under the main topic "Fuels, Synthetic, Liquid", at page 1520 et seq., the huge resources of natural gas have been recognized to be a source for conversion to liquid fuels as well. In general, the proven technology to upgrade methane, the main component of natural gas, is via steam reforming to produce synthesis gas, CO+H.sub.2. Such a gas mixture is clean and when converted to liquids produces fuels substantially free of heteroatoms such as sulfur and nitrogen. There are pathways from a synthesis gas which can be taken, and which have been commercialized, to produce liquid fuels from natural gas. One such pathway involves coupling with Fischer-Tropsch technology to produce fuel range hydrocarbons directly or upon further processing.
For instance, a SHELL OIL process, parts of which are described in U.S. Pat. Nos. 4,587,008 and 4,628,133 to Minderhoud et al., and more fully described in the September 1989 issue of Process Engineering at page 21, involves modifying a Fischer-Tropsch process to produce heavy paraffins at an intermediate stage from natural gas. The purpose of this modification is to improve the end product resulting from further processing of the paraffins, by eliminating the formation of certain undesirable length hydrocarbon chains.
Referring to FIG. 1, there is shown a schematic illustration of the Shell Middle Distillate Synthesis (SMDS) process developed by Shell Oil Company, which uses remote natural gas as the feedstock. FIG. 1 shows a simplified flow scheme. The two-step process involves Fischer-Tropsch synthesis of paraffinic wax called the heavy paraffin synthesis (HPS). The wax is subsequently hydrocracked and hydroisomerized to yield a middle distillate boiling range product in the heavy paraffin conversion (HPC). In the HPS stage, wax is maximized by using a proprietary catalyst having high selectivity toward heavy products and by the use of a tubular, fixed-bed Arge-type reactor. The HPC stage employs a commercial hydrocracking catalyst in a trickle flow reactor. The effect of hydrocracking the light paraffins is shown in FIGS. 2 and 3. As shown in FIG. 2, the product composition as a function of carbon number for the Shell Middle Distillate Synthesis process following HPS is much broader than what is obtained after the HPC step. The HPC step allows for production of a narrower range of hydrocarbons which is generally not possible with conventional Fischer-Tropsch technology. This aspect is shown in FIG. 3. Shell's two-step SMDS technology allows for process flexibility and varied product slates. The liquid product obtained consists of naphtha, kerosene and gas oil in ratios of from 15:25:60 to 25:50:25, depending on process conditions. High quality gas oil and kerosene are obtained. The products manufactured are predominantly paraffinic, free from sulfur, nitrogen and other impurities and have excellent combustion properties. The SMDS process has also been proposed to produce chemical intermediates, paraffinic sulfas, and extra high viscosity index (XHVI) lubeoils.
However, there is no teaching that producing heavy paraffins as an end product is beneficial, or that stopping the process after producing heavy paraffin is beneficial. The heavy paraffins, or wax products, produced by the modified Fischer-Tropsch synthesis process are instead immediately refined as part of a continuous process at the same location to produce middle distillate fuel oils which are low in methane and ethane content. These middle distillate fuel oils have a much greater economic value than the paraffins produced during the process and are therefore more desirable to produce as an end product. These middle distillates are also more flammable and cause more environmental damage when they are spilled.
Paraffin wax, by comparison, is non-volatile, floats on water and does not disperse or boil from its solid form at ambient temperatures. It is also considered to be of less economic value and to have few end uses as fuel. The SHELL process is the only one in the fuel production industry which considers the synthesis of paraffins to be beneficial, but solely for the purpose of further refining the paraffins in a continuing process to produce greater quantities of gasoline more efficiently.
As taught by U.S. Pat. Nos. 1,439,171, and 4,125,566, and others, heavy paraffins produced by distillation and Fischer-Tropsch synthesis are considered undesirable by-products of these fuel production processes.
Paraffin wax is simply a longer straight hydrocarbon chain than natural gas, (the shortest chains) and oils (middle length chains). Paraffin wax can be distilled to produce natural gases and oils, or added to other feedstocks to enrich the amount of hydrocarbons present or improve the quality of the refined oil. However, heavy paraffins have little use as a fuel without further refining.
Waxes alone are never transported from remote locations closer to local end user sites for fuel production refining. Paraffin waxes have been disclosed to be combined with oils and pumped through pipelines, as in U.S. Pat. Nos. 3,458,846 and 3,804,752. Paraffin waxes are also taught to be combined or recombined with local feedstocks to improve the quality of the end product, such as in U.S. Pat. Nos. 3,821,104, 3,308,052 and 2,917,375. Waxes used in these processes are often the by-products created in the initial refining.